1. Field of the Invention
The invention relates generally to power hand tools, such as rotary hammers and hammer drills for example, and more particularly to clutch arrangements for limiting torque in such tools.
2. Reference to Prior Art
Rotary percussive tools such as rotary hammers and hammer drills are capable of combining tool bit rotation with percussive action to drill holes in concrete and masonry. To prevent motor overload or burnout resulting from excessive torque loads within the tool, such as can result if the drill bit becomes stuck in a workpiece, it is known to provide rotary hammers and other power tools with slip clutch arrangements. Known slip clutches include driving and driven members that engage one another under the influence of a spring and that slip relative to one another when subjected to an overload torque sufficient to overcome the spring.
In one known slip clutch design conjugate teeth are embossed on the opposing faces of two clutch members, and the clutch members are spring biased into engagement to transmit torque between a driving member and a driven member such as an output shaft. When the tool is subjected to an overload torque, the spring force is overcome to permit the conjugate teeth to slip in a ratcheting action. An example of that type of slip clutch is illustrated in U.S. Pat. No. 4,719,976. That patent illustrates a hammer drill including a driven gear and a tool spindle having a flange. The driven gear and the flange are provided with corresponding lugs or teeth on their opposing faces and are spring biased toward one another. The teeth on the driven gear and the spindle flange slip relative to one another when subjected to an overload torque. Examples of similar slip clutch arrangements are provided in U.S. Pat. Nos. 4,446,931, 4,529,044 and 4,487,272.
Another known slip clutch arrangement used in power tools includes ball elements for transmitting torque between driving and driven members. In ball element clutch designs the ball elements are seated in corresponding detent depressions in the driven and driving members. When an overload torque is reached, the ball elements ride up out of the detent depressions in one of those members in a ratcheting action to decouple the driving and driven members. An example of a ball element clutch used in a power tool is provided in U.S. Pat. No. 2,566,183. That patent illustrates a clutch mechanism having a pair of clutch members and a spring for biasing those members together. The opposed faces of the clutch members have conical detent depressions, and balls are seated in corresponding pairs of depressions to transmit torque between the clutch members. When an overload torque is reached, the balls ride up out of successive depressions in one of the clutch members in a ratcheting action until normal torque loads are applied. Rotary hammers including ball element clutches are also illustrated in U.S. Pat. Nos. 3,828,863and 3,720,269.